In recent years, as an ink composition charged in a ballpoint pen, water-based ink compositions using an aqueous medium as a dispersion medium (water is a main solvent) are commonly used in consideration of environmental concerns and safety for humans. Among them, the water-based ink composition having a low viscosity solves handwriting bleeding by allowing to have a shear thinning viscosity.
A shear thinning viscosity-imparting agent includes polysaccharides such as xanthan gum (e.g., see Patent Document 1).
However, the shear thinning viscosity-imparting agent sometimes inhibits the dry up resistance. Therefore, in order to enhance the dry up resistance, an attempt has been made to combine additives such as a wetting agent, e.g., a water-soluble organic solvent and a solid wetting agent, e.g., urea with the shear thinning viscosity-imparting agent to suppress the drying.
However, it cannot be said that an effect of drying suppression is sufficient, and poor handwriting such as blur easily occur due to easy increases in the ink viscosity by the addition of the solid wetting agent.
Also, when the urea is added excessively, water is evaporated from a writing edge and a concentration of the water-soluble organic solvent is increased to precipitate a solid content at the writing edge. A so-called flower spot phenomenon, and the appearance worsens and occurrence of blur.
If the water-soluble organic solvent and the urea are added in large amounts, when the writing edge is left to stand facing downward in a humid environment, running occurs.
Although there are many procedures to enhance the dry up resistance in the ink composition as described above, these procedures sometimes have led to a reduction in other performances required for a ballpoint pen, e.g., handwriting blur and running have occurred.
Further, in the case of the ink composition charged and used in the ballpoint pen (retractable ballpoint pen) having a structure in which a ballpoint pen tip is always opened to the atmosphere when the ballpoint pen is not used for writing and requires no cap, the dry up resistance is particularly important.
Patent Document 1: Japanese Published Patent Application Sho-59-74175 A